Self-locking fixing systems known at present are generally based on a permanent radial deformation of the nut before mounting, which produces a torque resisting unscrewing by friction on the thread of the screw. Such systems have however drawbacks.
In fact, the friction is manifested during tightening and causes a resistant torque which introduces a considerable dispersion factor in the relationship between the mounting torque and the effective tension in the assembly. Moreover, the locking torque against unscrewing drops under the action of the first tightening, so that in order to guarantee a sufficient locking torque in service, it is necessary to attain a level of deformation of the nut which induces a relatively high torque resisting tightening.
It will finally be noted that, to the extent that there is no veritable locking of the nut on the screw, possible unscrewing during service, for example under the action of heavy vibrations, may not be completely impossible.